Gas turbines and the exhaust gas diffusers used for these have been known at the latest from the prior art. For example, an exhaust gas diffuser with a comparatively large opening angle of 10° and more is known from DE 198 05 115 A1. This rather large opening angle is achieved by provision being made in the center of the diffuser passage for an axially extending guiding body for extending an otherwise short gas turbine hub. By using the guiding body, the exhaust gas diffuser is formed as an annular diffuser. Larger regions of backflow zones aft of the gas turbine hub are consequently avoided, which has an advantageous effect upon the efficiency of the exhaust gas diffuser. The fact that the guiding body is comparatively long and on account of its length therefore has to be supported by means of additional struts is disadvantageous, however. Furthermore, the aerodynamic influences of the support struts are disregarded.
The known short gas turbine hubs mostly terminate directly aft of the turbine-side bearings of the gas turbine rotor. They have particularly large backflow zones, however. Nevertheless, the short gas turbine hubs are also particularly cost effective.
Also, an exhaust gas diffuser, which on the inside has an annular guiding element which is concentric to the outer wall, is known from EP 1 970 539 A1. The guiding element is designed in this case in such a way that a nozzle passage is formed between outer wall and guiding element, with the aid of which nozzle passage the near-wall flow can be accelerated. As a result, it is possible to avoid near-wall flow separations downstream of the guiding element. Influencing of the flow in the center of the exhaust gas diffuser, where backflows can occur, is not possible, however, with the aid of the guiding element.
Furthermore, U.S. Pat. No. 5,209,634 A1 discloses a steam-turbine diagonal diffuser with variable hub geometry for adjusting the diffuser cross section through which flow can pass.
The aim also exists of avoiding as far as possible the backflow zones located aft of the gas turbine hub, or of minimizing their extent, so that even during partial-load operation of the gas turbine high efficiency of the exhaust gas diffuser can be achieved and high operational reliability can be ensured. In the case of backflow zones reaching too far downstream, there is the risk that these can reach a boiler arranged downstream of the exhaust gas diffuser, which significantly degrades its principle of operation. Also, in the case of afterburners which are installed there, these would lead to a flashback, as a result of which the combined operation of gas turbines and afterburners is severely limited.